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Heat Exchanger 



.1 



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Heat Exchanger 

Written By: Charles Piatt 



TOOLS: 



PARTS: 



Drill (1) 

Picture frame clamps (1) 

SawM) 

Screwdriver (1) 

Thermometers (2-3) 

Digital thermometers are best. 

Vised) 



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Aluminum foil (1) 

Heavyweight foil is easier to work with. 

Hardwood (1) 

Double-sided tape (100 feet) 

Foam board insulation (1) 
from a home improvement store 

Masonite (1) 

or ABS plastic sheet or 2" plywood and 

adjust measurements 

DC fans (2) 

With DC fans you also need a power 
supply. This can be a simple adapter, 
but a better option is a variable DC 
voltage source, such as a model railroad 
speed controller, which lets you find the 
optimal setting for heat exchange. You 
can also use AC fans, but then speed 
control is harder. 

Epoxy (1) 
Fast-setting 

Wood screws (60) 
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Heat Exchanger 



SUMMARY 

An ideally energy-efficient house should be tightly sealed, to hold cool air inside during the 
summer and keep it out during the winter. The trouble is, we need to circulate some fresh air 
to remove odors, bring in oxygen, and reduce the risk of mildew and mold growth. 

Is there a way to move air in and out of a house while minimizing the heat moving in and 
out? 

One simple gadget can do it: a heat exchanger, aka "heat recovery ventilator." Instead of 
letting air run in and out freely, the exchanger uses two small fans to draw incoming and 
outgoing air through parallel interleaved ducts. The two flows don't mingle, but heat passes 
between them through the thin metal walls of the ducts. 

In the winter, warm air going out through the heat exchanger gives up its heat to cold air 
coming in, and during the summer, cool air blown out through the heat exchanger steals heat 
from hot air coming in, so that by the time the incoming air enters the house, it isn't hot 
anymore. 

Heat recovery ventilators are cheap to run, because they only contain a couple of fans. But 
they can be costly to buy, retailing for $450 and up. 

Here's how you can build your own for radically less, $50 to $100, depending on how many 
materials you already own. This is my first design attempt, and it works, but I don't pretend 
that I optimized it. Feel free to make it better. 

I am very grateful to MAKE intern Eric Chu for tackling the hard work of fabrication and 
testing, using plans that I drew. 

Design 

These are the crucial design features to get the most out of a heat exchanger: 

Interior panels should have maximum surface area relative to volume. Panels should be made from thin, thermally 
conductive metal. Incoming air and outgoing air must move in opposite directions. 

Since aluminum conducts heat very efficiently, I decided to make the panels from aluminum 
foil stuck to wooden frames, with holes drilled in the edges of the frames to allow air to pass 
through. Cheap computer fans are fine since they're quiet and don't use much power. Since 



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Heat Exchanger 

this unit is merely providing some moderate ventilation rather than heating or actively 
cooling roomfuls of air, the flow rate can be low. 

You might assume that slower airflow allows greater opportunity for heat transfer between 
air going out and air coming in. Theoretically, this should be true, but in practice other 
factors play a part, such as heat penetrating or escaping from the box containing the unit. 
When we tested our exchanger on the cold night air, we found that higher fan speeds 
actually warmed the incoming air more. Perhaps this is because faster-moving air lengthens 
the temperature gradient along the heat-transfer path and keeps the box containing the unit 
from getting cold. 

Where, exactly, is the sweet spot? I suggest you build the unit and adjust the fan speed 
experimentally to find out. 



Step 1 — Cut and drill the hardwood. 




• Cut the 3 A" square wood into 18 
lengths of 12" and 18 lengths of 24". 
The shorter pieces should be 
exactly 12", but it's OK if the 
thickness of saw cuts causes your 
24" lengths to be slightly short. 

• Starting 1-1/8" from one end of 
each of the 12"sections of wood, 
drill 8 centered holes 5/8" in 
diameter, spaced 3" apart. Reduce 
the diameter if your wood is 
undersized. 



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Page 3 of 7 



Heat Exchanger 



Step 2 — Make the frames. 






• Use the hardwood pieces to make 
9 rectangular frames, with the 
holes in the 12" pieces running in 
and out of the frame, rather than 
front to back, and positioned upside 
down relative to each other. Use 
fast-setting epoxy with 4 picture- 
frame clamps to hold the corners. 
Number the frames to identify 
them. 



Step 3 — Make the end panels. 



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• Cut 2 pieces of masonite (or ABS, 
or plywood), each measuring 
12"x12 3 /4". Drill each piece with 
columns of 5/8" holes. 

# The holes in your frames must 
align exactly with the columns of 
holes in the end panels. To make 
sure that they do, clamp a frame to 
the panel in each column position, 
and then drill the holes into the 
panel through the holes in the 
frame, to make them match. 



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Heat Exchanger 



Step 4 — Apply foil. 

• Cover both sides of frames 2 through 8 with rectangles of foil, using double-sided tape. 
Cover frames 1 and 9 with foil on only one side, the inward-facing side. 

• NOTE: Small air leaks around the edges of the foil are not very important, but tears 
in the foil will allow too much leakage. 



Step 5 — Install the panels. 




• Attach frame 1 along one side of an end panel, matching the holes and facing the foil 
inward. Use three 1 "—1 Vi" #6 flathead wood screws at top, middle, and bottom. Similarly 
join the other end panel onto frame 1, upside-down relative to the first end panel. 

• Then attach all the remaining frames into place, orienting and aligning their holes as you 
did with frame 1 . When you get to frame 9, its foil side should face inward. 

Step 6 — Enclose it. 

• The hard part is over. Now box up your frames by cutting top, bottom, and side panels, 
and taping them into place. For our test version we just used foam board, but for 
something sturdier to install in a house, screw or glue masonite panels, then clad them 
with foam board for insulation. 



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Heat Exchanger 



Step 7 — Mount the fans. 




• Make 2 manifolds for the fans, one sealed over the top set of holes, and one over the 
bottom set, on the end of the box that will sit inside the house. One fan will blow inward, 
while the other blows outward. We made a single trapezoidal hood by cutting, bending, and 
taping foam board, and then used another piece of foam as a divider inside. 

• Fit the fans snugly in the manifold ends. We taped around them, for air tightness more than 
to hold them in place. That's it! 

Step 8 — Time for Testing! 

• First, measure the temperature outside and inside. For a good test, you need at least a 

20 °F difference between the two. Place the heat exchanger in an open door or window and 
block the open space around it to prevent stray air currents. 

• We aimed ours out a screened window masked with more foam board. The screen may 
have increased the amount of exhaust air that doubled back and was drawn inside again. 
To minimize this, you could divert the outgoing air off to the side with a small baffle or 
shroud. 

• Run the fans, and measure the temperature of the incoming air. It should gradually move 
closer to the inside temperature. If you have an adjustable fan-speed control, try different 
speeds to find the one that warms incoming air the most. 



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Heat Exchanger 

Step 9 

• Our exchanger worked best when the fans ran at 100%. Here are our results at that speed: 

• Trial 1 -- Outside temperature: 42.6° F; Inside temperature: 71. 4° F; Incoming air 
temperature: 62.2° F 

• Trial 2 -- Outside temperature: 45.3°F; Inside temperature: 67.1 °F; Incoming air 
temperature: 58.2° F 

• Trial 3 -- Outside temperature: 41.0°F; Inside temperature: 67.4° F; Incoming air 
temperature: 58.1 °F 

• Your heat exchanger will be especially useful in a home that's fairly airtight, with good draft 
seals around doors and windows. In the summer, it can save power by providing 
ventilation without losing the coolness from your recirculating air conditioner. In the winter, 
it's ideal in conjunction with an unvented gas heater. This type of heater retains all its 
thermal energy inside the house, while the heat exchanger gets rid of carbon dioxide and 
brings in oxygen. Together they form a perfect partnership. Download the heat exchanger 
plans at http://www.makezine.com/18/heatexchanger . 



This project first appeared in MAKE Volume 18 , page 56. 

This document was last generated on 2012-10-30 06:41:50 PM. 



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